High performance analog-to-digital converters (ADCs) are now widely used in many applications, including RF receivers (e.g., radar) and electronic countermeasures, communication systems, test instrumentation and others, that handle large dynamic ranges of signal amplitudes of a high data rate signals. Ideal ADCs have equally spaced levels of voltage references against which the input signal is compared. Ideal ADCs transfer energy from the frequencies of the input signal or signals to other frequencies as a result of the inherent non-linearity of their transfer function. The transferred energy is often referred to as spurs, as they show up as spikes in a spectrogram of the device output when the input is a tone. Most ADCs suffer additional non-linearities. One particular problem in such high performance ADCs is differential non-linearity (DNL) errors. DNL error is generally defined as the difference between an actual transfer function step width of an ADC and the ideal value of 1 least significant bit (LSB), and is often due to mismatches in the ADC's resistance ladder providing threshold reference voltages and its comparator circuits. Spurs can cause a significant degradation for some systems, especially where a large signal is present and the system must reliably detect much smaller signals at the same time. The spurs or distortion can cause false detections or cause missed detections. The electronics industry is constantly striving to improve the spurious free dynamic range (SFDR) of ADCs. A receiver with excellent SFDR is able to detect small signals in the presence of much larger ones. Non-linearities, for example DNL errors, effectively decrease a receiver's SFDR rating.
A well-known technique called dithering is often required to maximize SFDR. Dithering is the process of adding an uncorrelated signal, such as pseudo random noise (PRN) or broadband noise, to a desired analog signal prior to the analog input gate of the ADC. A common approach to creating dither is to use a noise or thermal diode whose output is summed with the wanted signal prior to digitization. Although the injected dither does not eliminate the errors, it whitens the resulting errors, spreading the spurs across a wideband of frequencies with much less power at any frequency. Without dither an input signal constantly is quantized at a particular portion of the dynamic range with some given DNL errors of the ADC, thereby repetitively providing the same error. The repetition forces the spurious signals to be at a set of frequencies and amplitudes for a given input. Adding dither to the input results in the combined signal being converted across a wider set of reference voltages interacting with different ones as the dither varies, even when the wanted inputs signal has a constant waveform. Adding dither improves the resolution and linearity of the conversion by effectively smoothing the quantization errors of the ADC's transfer function. However, while spurs are reduced, a commensurate increase in the noise floor occurs as adding the dither is equivalent to adding noise to the wanted signal. Many conventional systems simply accept degradation of the noise floor to improve SFDR or they sub-optimize SFDR to avoid the additional noise. FIG. 1A illustrates a prior art embodiment of a SFDR maximization, wherein a digital PRN generator 10 generates a random digital signal that is converted to an analog dither signal by a high dynamic range digital-to-analog converter (DAC) 12 coupled to a summer 14, which adds the analog dither signal to an analog input signal 14 before the dithered analog signal is digitized by ADC 16. The “known” random digital signal is subtracted from the converter response at digital subtractor 18. This is a more expensive process for dither creation than a simple diode and will not be 100% random. Further, if a system has multiple ADCs whose results are to be combined (e.g. a phased array radar), where each ADC requires dither then the dither source must produce random dither for each ADC that is further uncorrelated with the all the other dithers created. To be random for one ADC and uncorrelated to many dither sources is a challenge to a PRN generator. There is interest in putting an ADC at each element of a phased array (enabling element level digital beamforming) but it is difficult if not impractical to provide random dither to each ADC that is not correlated with any of the other dither digitally created for other ADCs.
FIG. 1B shows another common technique for spur reduction, wherein a wideband non-correlated signal is generated using a thermal noise source 20, and then added to the analog input signal by a summer 14. Depending upon on how much noise must be injected, signal-to-noise ratio (SNR) of the ADC 16 may be unduly sacrificed.